The conversion of aromatic methyl groups to the benzaldehydes (the simplest example being the oxidation of toluene) may be achieved by a number of methods. All methods are included in a group of reactions termed oxidations. Such oxidations may be accomplished by a number of reagent chemicals. Factors which differentiate the reagent chemicals from the standpoint of fitness for a particular application include cost, selectivity, efficiency, waste materials produced and the desired scale of reaction.
Typical reagent chemicals employed for the oxidation of aromatic methyl groups include metals at a high oxidation level such as cobalt, manganese, chromium, cerium, iron or copper. Non-metallic reagents include persulfate and peroxides, but these may frequently require a metallic catalyst. Air or oxygen may also be used as an oxidant, but the use of a metallic catalyst is almost always required.
A number of industrial processes have been described for the oxidation of aromatic methyl groups, with most employing a metal catalyst and an inexpensive oxidant such as air, oxygen, or ozone. Some of these processes employ a solvent under batch type conditions, while others are run without a solvent in a continuous type reactor.
The oxidation of the methylcatechols is a special instance in the family of reactions briefly described above. The use of a metal catalyst and either air or oxygen under appropriate conditions does result in formation of the desired benzaldehyde. However, significant byproduct information is also encountered under the test conditions employed. To date, for the methylcatechols, no metal catalyzed air or oxygen oxidation has resulted in an industrially viable process. This is borne out by prior descriptions of oxidations of methylcatechols and their analogs. U.S. Pat. No. 4,335,263 describes a process wherein the oxidation of methylcatechol analogs to benzaldehydes is achieved through the use of molecular bromine under radical conditions followed by further oxidation with dimethylsulfoxide. Another prior process utilizes molecular chlorine, again under radical conditions, to obtain the corresponding benzoic acids (after hydrolysis of the trichloride intermediate) rather than the benzaldehydes.